This invention relates to devices used for testing electronic components in a mother board/daughter board configuration and the general application of connecting two printed circuit boards (PCBs) in parallel configuration for improving the speed, purity and accuracy of electrical test signals used during component testing.
Electronic components such as microprocessor devices, multi-chip packages, memory chip packages, field programmable gate arrays (FPGAs), dynamic random access memory (DRAM) and the like are utilized in packages such as ball grid arrays (BGAs) and chip scale packages (CSPs). These packages are well known in the art and are being used more and more frequently because they offer greater circuit density and higher pin counts than other packages such as quad flat packs (QFPs) and pin grid arrays (PGAs). Packages such as BGAs and CSPs also offer better electrical performance due to the shortened circuit path from the device die to the printed circuit board.
Prior to their being installed onto PCBs, electronic components such as those mentioned above must be thoroughly tested to ensure that the components are electronically correct, and are functioning properly according to specifications. The same is true of other electronic components such as the PCBs that are used to test other components. In an idealized testing regime the goal is to have the component (whether the xe2x80x9ccomponentxe2x80x9d is a PCB or, for example, a BGA) behave as if it were directly connected to the adjacent PCB and to eliminate any possible fault indications caused by the testing process itself. For instance, in many components certain switches and circuits are designed to react in a specific way to electrical phenomena such as a voltage drop. However, if during the testing process a voltage drop occurs unintentionally, for instance, due to an imperfect signal, a switch may react to the drop in voltage as it normally would. While this may be the proper reaction, in the context of the test it may be interpreted as a fault reaction. In other words, the test regime did not account for a voltage drop due to an imperfect signal. As a result, an error indication is noted where in fact there was no error.
Signal errors and imperfections could be eliminated if the components were directly connected to the PCBs such as by soldering. That, however, is obviously impractical for a test process since the component could not be readily separated from the test board. The test equipment must therefore be designed to minimize signal errors and imperfections while allowing for economical testing processes.
By way of illustration, and with reference to one typical interface that is used to test components, devices called xe2x80x9ctest socketsxe2x80x9d are used as part of the testing procedure and are intended to help ensure that test results are not flawed by errors that occur during the testing process as a result of the test equipment itself. The test sockets define an interface device that holds the component (such as a BGA) and allows the component to be interfaced with a test machine that is programmed to send electric test signals to the component. In this testing procedure the component being tested is referred to as the xe2x80x9cdevice under testxe2x80x9d or xe2x80x9cDUT.xe2x80x9d There are a wide variety of test sockets in use today. Nonetheless, in most instances the test socket comprises a box like compartment that is designed to hold the integrated circuit package such as a BGA in a desired position. For purposes of illustration, the following description refers to a BGA package. It will be understood that the description applies just as well to other packages.
The test socket typically has an internal compartment or window into which the BGA fits. The lower portion of the compartment is called a probe plate. It has a xe2x80x9cfootprintxe2x80x9d of holes drilled through it that matches the array of test points on the BGA. A spring probe is retained in each hole in the probe platexe2x80x94there is a spring probe associated with each test point on the BGA. Each spring probe extends from the interior portion of the test socket in the compartment where the BGA is held through the bottom of the socket.
The test socket is used in connection with a printed circuit board (PCB) that is specially designed solely for testing the particular BGA. This special PCB has many names, for instance xe2x80x9cload boardxe2x80x9d or xe2x80x9cDUT board,xe2x80x9d the later meaning the xe2x80x9cdevice under test board.xe2x80x9d The later naming convention is used herein. The DUT board has only one purpose, and that is to facilitate testing of a specific component. The DUT board electrically interfaces the component under test with the test machine that sends signals to the component.
As noted, the DUT board is specially designed to test a specific component such as a microprocessor die packaged in a BGA. The board typically has many different circuits and it may have many different electronic components. These vary depending upon the device under test and other factors relating to the testing regime. But the DUT board also always has a footprint of electrical pads on one outer surface of the board that matches the footprint of the spring probes that extend through the bottom of the test socket. In practice, the footprint pattern is drilled through the DUT board according to the array pattern of test points on the device under test. An electrically conductive pad is then deposited on one surface of the board according to well known PCB manufacturing techniques. Each pad is electrically connected through the associated hole and associated traces to other components and ultimately to the test machine interface.
Just as there are many types of test sockets, there are also many types of DUT boards. In some instances multiple DUT boards may be used in combination to test a component. In other instances, multiple DUT boards or other printed circuit boards may be combined in order to provide an interface with a testing machine so that one may vary the combination of boards and/or sockets.
In instances where multiple PCBs are interfaced, or stacked, the multiple boards are oriented adjacent one another. The interface between the two boards may be made with a spring probe holding plate oriented between the boards or through connectors. The spring probes probe respective associated pads on the facing surfaces of the board in order to transfer test signals from the test machine through selected circuits on the boards. But the interfaces between the boards results in an undesirable increased distance between the boards.
For purposes of further illustration, a typical test process involving adjacent PCBs will be described. In the following example the adjacent boards are referred to as the mother board and the daughter board. It will be understood that this example is for illustration only, and that a similar illustration may be made with other board to board interconnections, and with board to socket interconnects. Nonetheless, in this board to board situation mother board is mounted adjacent the daughter board such that the one end of each spring probe held in the probe plate is physically urged against a test pad on the mother board, and the opposite end of each probe is in contact with the associated pad on the daughter board. This compresses the spring probes to insure good electrical contact between the pads and the spring probes. The two adjacent boards are typically bolted to one another with the probe alignment plate sandwiched in between, resulting in physical and electrical contact between the boards through the spring probes. In this way there is an electrical connection established from the test machine through a variety of traces and components in the mother board, through the spring probes, and to the associated test points on the daughter board. This compressive load is referred to as xe2x80x9cpre-loadxe2x80x9d compression.
The paired boards are then connected to a test machine, and the test machine can then begin sending test signals according to a preprogrammed testing routine to the boards to determine whether the boards meet specifications.
The electrical characteristics of the PCBs are important factors in the testing process since they are a potential source of irregularities in the electrical signals being transmitted between boards. Ideally, the electrical signals transmitted into between the boards should be flawless (i.e. xe2x80x9ctransparentxe2x80x9d) and free of electrical irregularities, often called parasitic components. By eliminating parasitic components and providing electrically pure signals, the test results obtained are a true measure of the performance of the component under test. On the other hand, when the test signals are contaminated with parasitic components, the test results may not provide the desired level of certainty in the performance of the device. While there is software available to correct for parasitic components, the better method is to eliminate such components.
As one example of a signal imperfection that can cause a fault error that should not have occurred, consider the situation described above with regard to voltage drops caused by signal imperfections.
The electrical dynamics associated with PCBs are highly complex, and there are several sources of electrical signal impurities such as capacitance, inductance, resistance and signal interference such as cross talk and distortion. One of the main sources of signal impurity is found in the spring probes that interconnect the boards. Generally speaking, as the electrical path from the test machine (or signal generator) gets shorter, the parasitic components in the signal are decreased. For example, capacitance and inductance are related to the length of the spring probe (among other factors). By decreasing the length of the spring probes, and therefore the length of the electrical path, capacitance and inductance are reduced and signal interference between adjacent pins is likewise reduced. It is beneficial, therefore, to shorten the probes as much as possible.
As another example of a signal imperfection, a condition called xe2x80x9cskin effectxe2x80x9d is a problem that occurs with high frequency test signals. Skin effect relates to the properties and electrical characteristics associated with electrical signals travelling up the outer surface of a spring probe and creating unusual, and undesired interference between the signals travelling through adjacent pins. The length of a spring probe is one factor that contributes to greater skin effect problems.
Spring probes can therefore be a significant source of signal impurities and can adversely effect test results. The spring probes are nonetheless necessary components since they ensure 100% electrical connection between the pads on adjacent boards. There is a need, therefore, for apparatus for improving the electrical characteristics of test signals sent through mother/daughter combinations while maintaining 100% compliance in connectivity.
The present invention provides an apparatus and method for significantly improving the speed at which test signals are transferred between PCBs, and for providing a signal that has improved electrical purity by reducing parasitic components such as capacitance and inductance. According to the present invention, spring probes are retained in the holes drilled in the board rather than in a probe alignment plate.
The present invention involves the use of multiple adjacent boards, for example a mother board and a daughter board, to either test a component that is retained in a socket attached to the daughter board or, for example, to interconnect one or both of the boards. In this case, spring probes are retained in the holes drilled in the mother board and make contact with associated test points on the facing surface of the adjacent daughter board, or vice versa. The spring probe is contained within the board itself. This is accomplished by forming the test pads on one or both of the boards as holes that electrically interface with spring probes received within the holes. For instance, the holes may be formed in the motherboard. The ends of the spring probes make physical and electrical contact with associated test points on the next adjacent board, the daughter board, which is held in far closer proximity to the motherboard than would be possible with a standard probe receiving plate or connector. In other words, the distance between the upper surface of the motherboard and the facing surface of the daughter board is significantly reduced. This eliminates a substantial distance through which test signals travel and significantly decreases parasitic components. The same principles may be applied to combinations of boards where each board has spring probes inserted into holes in the boards. Moreover, such boards may be used with sockets.
Alternately, a board using spring probes held within holes in the board may be used in combination with an adjacent dielectric material to modify the electrical characteristics sent through the probes.
The electrical connection between the spring probe and the modified xe2x80x9ctest padxe2x80x9d on the board can be established in several ways using standard trace techniques. As is typical with most PCBs, the board used with the present invention has multiple levels. The traces may connect to the spring probe holes at any level in the board. For instance, each hole in the footprint can be filled with solder, then partially drilled to leave a solder plug in the bottom of the hole. The solder plug, deep within the hole, replaces the pad on the outer surface of the board. Alternately, each hole may be filled at the lower end with a solder ball. Finally, the outer surface of the spring probe itself can be used to establish an electrical connection with a hole that is lined with electrically conductive material but open on opposite ends. To further reduce the adverse effects of parasitic components, ground planes may be placed around the trace in adjacent vertical layers of the board or horizontally next to each signal line.